Wordpress import issues - "Internal; server error" when trying to import .xml

I have persistently had the same issue over the past week; when I try to import my exported .xml file from my old website, and onto the new one, I get a 500 internal server error.

I have tried various suggested fixes, changing permissions, contacted my hosting (I'm on a VPS so they didn't really help). However one fix was suggested but I am unable to implement it, it has been suggested that I convert the file to a .gz via gzip(?). I have stuggled to do this, and haven't worked out a way to convert it, I am using Windows.

Regards